How to choose a transformer local line monitoring system? Comparison of Ultrasonic, HF and UHF Solutions

• ultrasonic method: It locates the discharge point by detecting ultrasonic signals (20 kHz–300 kHz) generated by partial discharges. The sensor is mounted on the outer wall of the oil tank; it is sensitive to internal discharges but less sensitive to corona discharges.
• high-frequency current method: Detects partial discharge pulse currents (30 kHz–30 MHz) using a high-frequency CT mounted on the ground wire; offers the highest sensitivity and can capture even the faintest partial discharge signals
• UHF method: Uses antennas or probes to detect electromagnetic waves (300 MHz–3 GHz) emitted by partial discharge; offers the highest resistance to interference and is suitable for substations in environments with strong electromagnetic fields
• Multi-sensor fusion: Each of the three methods has its own strengths—ultrasound excels at positioning, high-frequency current excels at sensitivity, and ultra-high frequency excels at interference resistance; the three-in-one solution covers the widest range of scenarios.

1. Principles and Characteristics of the Three Detection Methods

2. Recommendations for Selecting Solutions Based on Specific Scenarios

2.1 Single-Sensor Solution—An Affordable Entry-Level Option

If you have a limited budget and only require basic partial discharge monitoring, the single-sensor ultrasonic solution is the simplest option. It is the easiest to install; simply attach the sensor directly to the tank wall. It is suitable for basic monitoring of distribution transformers rated at 35 kV or below. Its limitations include a lack of sensitivity to corona discharge and an inability to distinguish between different types of discharge.

2.2 Dual-Sensor Solution—The Best Value Option

Ultrasonic sensors and high-frequency current sensors are the most commonly used sensor combination. High-frequency current sensors provide highly sensitive signal detection, while ultrasonic sensors provide a reference for locating partial discharges. The two complement each other and cover virtually all common types of partial discharges in transformers. This combination is suitable for monitoring partial discharges in main transformers at 110 kV urban substations.

2.3 Three-Sensor Fusion Solution—Flagship Configuration

The three-in-one solution combining ultrasonic, high-frequency current, and ultra-high-frequency technologies represents the flagship configuration for partial discharge monitoring. Signals from the three sensors are fused and analyzed within the diagnostic unit: high-frequency current captures faint signals, ultrasound aids in localization, and ultra-high frequency filters out environmental interference. Suitable for hub substations rated at 220 kV and above, as well as applications with the most stringent requirements for partial discharge monitoring.

3. The Value of Multi-Sensor Fusion

A multi-sensor system is not merely a side-by-side display of data; the key lies in fusion. When two or more sensors detect partial discharge signals within the same time window, the reliability of the signal is greatly enhanced, and false alarms from a single sensor can be ruled out. At the same time, different sensors complement each other in their sensitivity to different discharge types: ultrasonic sensors are sensitive to internal air-gap discharges, high-frequency current sensors are sensitive to pulsed currents in ground loops, and ultra-high-frequency sensors are sensitive to high-energy arc precursor discharges.

By integrating diagnostic algorithms that combine the signal characteristics of three sensors, the system produces diagnostic conclusions that are more accurate and comprehensive than those from a single sensor—this is the core value of the multi-sensor solution.

4. Frequently Asked Questions FAQ

4.1 Q: Which of the three sensor solutions is best suited for older transformers?

A: The insulation in older transformers has deteriorated, making them more susceptible to partial discharge. We recommend using at least a dual-sensor system combining ultrasonic and high-frequency current sensors—the high sensitivity of the high-frequency current sensor in detecting partial discharge signals provides the most timely early warning. If the budget allows, a three-in-one solution offers even greater safety.

4.2 Q: Do ultrasonic sensors require a coupling agent?

A: During installation, a special coupling agent must be applied to eliminate the air gap between the sensor and the tank wall, ensuring proper acoustic coupling. Some systems use a magnetic mounting method, in which the coupling agent is pre-applied to the sensor’s contact surface, making it ready for use upon installation.

4.3 Q: Can UHF sensors and ultrasonic sensors be used interchangeably?

Answer: No. The two technologies operate on completely different physical principles, detecting different physical signals generated by partial discharges (electromagnetic waves vs. sound waves), and their frequency ranges do not overlap. In environments with strong electromagnetic interference, the interference-resistant advantages of UHF are irreplaceable; similarly, in terms of partial discharge localization, the precision advantages of ultrasound are equally irreplaceable.

4.4 Q: Where is the best place to install a high-frequency current sensor?

A: They are typically installed on the transformer core grounding wire, the grounding wire of the clamping device, or the neutral point grounding wire. Partial discharge signals captured at different locations originate from different sources; in a multi-channel system, sensors can be installed at multiple locations simultaneously, and the time difference in signal arrival can be used to help determine the direction of the discharge source.

4.5 Q: Should I choose the system with the most channels?

A: The number of channels determines how many sensors can be connected simultaneously. A 4-channel system is sufficient for most 110kV main transformers (e.g., 2 ultrasonic sensors + 2 high-frequency sensors). A 6-channel system is suitable for large transformers rated at 220kV or higher, or for scenarios requiring monitoring at multiple locations. The key is to balance system selection with budget; more channels are not necessarily better.

5. Summary

Select the appropriate sensor based on your requirements: choose ultrasonic sensors for positioning capabilities, high-frequency current sensors for sensitivity, and UHF sensors for interference resistance. A two-sensor combination meets most needs, while three-sensor fusion provides the most comprehensive protection.

Disclaimer: The content of this article is for technical exchanges and reference only, and does not constitute any form of procurement commitment or contract offer. Product technical parameters, configuration programs and prices are subject to the actual signed contracts and technical agreements. The technical data and cases involved in this article are from public information and engineering practice, if updated without notice.

Need advice on selecting an online partial discharge monitoring system for transformers? Contact Innotongda for a customized sensor configuration solution. Service Hotline: 13959168359 (also on WeChat).